1. Interference of frequency converter to microcomputer control board

In the control system using frequency converter, microcomputer or PLC is mostly used for control. In the process of system design or transformation, we must pay attention to the interference of frequency converter to microcomputer control board. Because the microcomputer control board designed by the user is generally of poor process level and does not conform to EMC international standards, the conduction and radiation interference generated after the adoption of frequency converter often leads to the abnormal operation of the control system, so necessary measures need to be taken.

(1) Good grounding. The grounding wire of strong current control system such as motor must be reliably grounded through grounding busbar, and the shielding ground of microcomputer control board is better to be grounded separately. For some occasions with serious interference, it is recommended to connect the sensor and i/o interface shielding layer with the control ground of the control board.

(2) Adding EMI filter, common mode inductor, high-frequency magnetic ring, etc. to the input power supply of microcomputer control board has low cost. It can effectively suppress conducted interference. In addition, in the case of serious radiation interference, such as GSM or PHS stations around, the microcomputer control board can be shielded by adding a metal mesh shield.

(3) Adding EMI filters to the input of the frequency converter can effectively suppress the conducted interference of the frequency converter to the power grid. Adding input AC and DC reactors L1 and L2 can improve the power factor and reduce harmonic pollution, with good comprehensive effect. In some cases where the distance between the motor and the frequency converter is more than 100m, it is necessary to add AC output reactor L3 at the frequency converter side to solve the leakage current protection caused by the output wire to the ground distribution parameters and reduce the external radiation interference. An effective method is to adopt the method of threading steel pipe or shielding cable, and reliably connect the steel pipe shell or cable shielding layer with the earth. Please note that when AC output reactor L3 is not added, if the method of steel pipe threading or shielded cable is adopted, the distributed capacitance of output to ground is increased, and overcurrent is easy to occur. Of course, in practice, only one or several methods are generally adopted.

(4) Electrically shield and isolate the analog sensor detection input and analog control signal. In the design process of the control system composed of frequency converter, it is recommended not to use analog control as far as possible, especially when the control distance is greater than 1m and is installed across the control cabinet. Because the frequency converter generally has multi-stage speed setting, switching frequency input and output, which can meet the requirements. If analog quantity control is not necessary, it is recommended to use shielded cable and realize remote one-point grounding at the sensor side or frequency converter side. If the interference is still serious, dc/dc isolation measures need to be implemented. Standard dc/dc module can be used, or v/f conversion, optical coupling isolation and frequency setting input method can be adopted.

2. Anti interference of frequency converter

When there are high-frequency impact loads near the power supply system of the frequency converter, such as electric welding machine, electroplating power supply, electrolytic power supply or the use of slip ring power supply, the frequency converter itself is easy to be protected due to interference. The following measures are recommended for users:

(1) Inductors and capacitors are added to the input side of the frequency converter to form an LC filter network.

(2) The power line of the frequency converter supplies power directly from the transformer side.

(3) If conditions permit, separate transformers can be used.

(4) When using external switching value control terminal control, it is recommended to use shielded cable when the connection line is long. When the control line and the main circuit power supply are buried in the trench, except that the control line must adopt shielded cable, the main circuit line must adopt steel pipe shielded threading to reduce mutual interference and prevent the wrong operation of the frequency converter.

(5) When using external analog quantity control terminal control, if the connection line is within 1m, use shielded cable connection, and implement one-point grounding on the inverter side; If the circuit is long and the field interference is serious, it is recommended to install a dc/dc isolation module on the side of the frequency converter or adopt the v/f conversion and the given mode of frequency command for control.

(6) When using external communication control terminal control, it is recommended to use shielded twisted pair and ground the shielding layer (PE) on the inverter side. If the interference is very serious, it is recommended to ground the shielding layer to the control power supply (GND). For RS232 communication mode, the control line should not exceed 15m as far as possible. If it is to be lengthened, the communication baud rate must be reduced accordingly. When it is about 100m, the baud rate that can communicate normally is less than 600bps. For RS485 communication, terminal matching resistance must also be considered. For the high-speed control system using fieldbus, the communication cable must be special cable and multi-point grounding mode to improve the reliability.

3. Power grid quality problems

In high-frequency impact loads such as electric welding machines, electroplating power supplies, electrolytic power supplies and other occasions, the voltage often flickers; In a workshop, when hundreds of capacitive rectifier loads such as frequency converters are working, the harmonic of the power grid is very large, which seriously pollutes the quality of the power grid and destroys the equipment itself. At least, it can not operate continuously, and at worst, it causes damage to the input circuit of the equipment. The following measures can be taken:

(1) In high-frequency impact loads such as electric welding machines, electroplating power supplies, electrolytic power supplies and other occasions, it is recommended that users add reactive static compensation devices to improve the power factor and quality of the power grid.

(2) In workshops with concentrated frequency converters, it is recommended to adopt centralized rectification and DC common bus power supply. It is recommended that users adopt 12 pulse rectification mode. It has the advantages of low harmonic and energy saving, and is especially suitable for occasions where frequent starting and braking, electric operation and power generation operation are carried out at the same time.

(3) The passive LC filter is installed on the input side of the frequency converter to reduce the input harmonic and improve the power factor. The cost is low, the reliability is high, and the effect is good.

(4) The input side of the frequency converter is equipped with an active PFC device, which has the best effect, but the cost is high.